1. Field of the Invention
The present invention relates to a tape driving control apparatus for use in a video tape recorder or the like.
2. Description of Prior Art
Referring to FIG. 1, there is shown a schematic construction diagram of a tape driving control apparatus for use in the FF (fast forward) condition of the conventional cassette type of video tape recorder (hereinafter referred to as VTR). In FIG. 1, a tape cassette 1 accommodates a magnetic tape 2 which is wound around reels 5 and 6. In the VTR, reel shafts 3 and 4 on the feed side and the winding side are adapted to be engaged with the reels 5 and 6 on the feed side and the winding side of the tape cassette 1. Also, reel motors 7 and 8 are provided to drive directly the reel shafts 3 and 4 on the feed side and the winding side. In the FF (fast forward) position, the magnetic tape 2 is wound around the reel 6 on the winding side from the reel 5 on the feed side. Frequency generators (hereinafter referred to as FG) 9 and 10 are also provided to detect the rotational speed of the reel motors 7 and 8 on the feed side and the winding side. The output signals of the FIGS. 9 and 10 are inputted into frequency-voltage converting circuits 11 and 12 on the feed side and the winding side, wherein the frequencies are converted into voltages. The voltage outputs of the frequency-voltage converting circuits 11 and 12 are added at the ratio of 1:1 in a mixing circuit 13. The output of the mixing circuit 13 is inputted into a speed comparing circuit 14 on the winding side for comparison with the reference voltage from a speed-setting reference voltage circuit 15. Then, the output of the speed comparing circuit 14 on the winding side is inputted into a reel motor driving circuit 16 on the winding side to control the speed of the reel motor 8 on the winding side. The magnetic tape 2 of the reel 5 on the feed side is wound, around the reel 6 on the winding side, at an approximately constant tape speed. On the other side, an instruction voltage from a back tension torque producing circuit 17 on the feed side is inputted into a reel motor driving circuit 18 on the feed side to cause a given motor current to flow to the reel motor 7 on the feed side so as to produce the torque in the direction of an arrow 19 so that proper back tension may be produced in the magnetic tape 2.
According to the above described conventional example, the tape speed of the magnetic tape 2 is slowest at the leading end and the trailing end of the magnetic tape and is fastest at the central portion of the magnetic tape 2 both in the FF (fast forward) position and the REW (reverse winding) position. The difference between them is so small that the tape speed is considered almost constant. However, one makes the tape speed fast so as to shorten the respective time of the FF (fast forward) and the REW (reverse winding), an extremely large force is applied upon the trailing end of the magnetic tape, thus resulting in the tape being broken due to the fast speed of the tape in the trailing end portion thereof. Accordingly, in the conventional apparatus, there was a limit to the fast tape speed, and there was a problem in that the respective time of the FF (fast forward), and the REW (reverse winding) could not be shortened. Also, since the magnetic tape speed of the FF (fast forward) is almost constant, the rotational speed of the reel motor 8 on the winding side in the tape leading end portion becomes extremely fast, thus resulting in noises from the reel motor on the winding side. Particularly, when a reel stand on the winding side is driven by the reel motor on the winding side through a reduction mechanism, such as gears, the noises produced by the reduction mechanism are a problem.
Furthermore, in the FF (fast forward) position, a constant current flows to the reel motor 7 on the feed side to produce torque in the direction of an arrow 19. The produced torque Ts of the reel motor 7 on the feed side is as follows: EQU Ts=Krs.times.Is (1)
wherein Krs: the torque production constant of the reel motor on the feed side. The tape tension T.sub.A of the exit from the reel 5 on the feed side is as follows: EQU Ts=T.sub.A .times.Rs (2)
wherein Rs: the radius of the tape wound around the reel 5 on the feed side. Since the produced torque Ts of the reel motor 7 on the feed side is constant, the tape tension T.sub.A of the exit from the reel 5 on the feed side increases in an inverse proportion to the decrease of the radius Rs of the magnetic tape 2 wound around the reel 5 on the feed side. As it passes each post, the tape tension increases as e.sup..mu..theta. (.mu. is a tape friction coefficient, .theta. is a tape winding angle), and the tape tension T.sub.B of the entrance of the reel 6 on the winding side becomes extremely large, and the load of the reel motor 8 on the winding side increases. The speed of the magnetic tape 2 must be slow or the winding operation cannot be effected in the worst case. Also, as the tension changes by the tape winding diameter ratio of the reel 5 on the feed side at the winding leading-end and the winding trailing-end, the tape tension changes by as much as approximately three times and the tape running condition becomes worse.